The "bundle" offer (five e-books for $25 USD) appears to apply to any five e-books or videos in Packt Publishing's large catalog. However, there are several pre-selected bundles ("tech playlists") available for developers wishing to purchase these books in one category. For example, here are some of the available pre-selected bundles:

The "list" prices for the above five PostgreSQL books would have totaled over $140. It was nice to pay only $25 for all five of them.

As part of the "Skill Up" promotion, reports on developers' salaries are available. The purpose of these appears to be for developers to determine the salary ranges associated with certain types of skills and then, in theory, a developer could purchase e-books and videos on those topics of interest (and associated with promising salaries) to learn those skills.

The "Skill Up" promotion appears to have just started, but I don't know when the "Skill Up" promotion ends.